--- a/sys/libiconv/iconv_ucs.c
+++ b/sys/libiconv/iconv_ucs.c
@@ -473,6 +473,17 @@
 		ucs4 |= *(src + i) & 0x3f;
 	}
 
+	/*
+	 * Reject overlong (non-shortest) encodings: the decoded value must
+	 * not be representable in fewer bytes than the width actually used.
+	 * Without this, e.g. "\xC0\xAF" decodes to U+002F ('/') and is
+	 * written into converted filenames, and 4-byte encodings of BMP
+	 * values make encode_surrogate() wrap below 0x10000.
+	 */
+	if ((w == 2 && ucs4 < 0x80) || (w == 3 && ucs4 < 0x800) ||
+	    (w == 4 && ucs4 < 0x10000))
+		return (0);
+
 	*utf8width = w;
 	return (ucs4);
 }